Sulfonated materials have been applied in a wide range of membrane applications due to their high electrical conductivity, good chemical stability, hydrophilicity and low fouling propensity. The fixed charged groups and hydrophilic nature not only endow sulfonated polymers with superior physicochemical properties for membrane applications but also significantly change the membrane formation process. Polyethylene glycol (PEG) has been widely used as an additive to produce membranes from polymer solutions by the non-solvent induced phase separation (NIPS) method. Its effects on sulfonated polyphenylenesulfone (sPPSU) solutions are therefore investigated and compared with those of polyphenylenesulfone (PPSU) solutions from different thermodynamic and kinetic aspects. It was found that (1) PEG could interact with sPPSU in the form of hydrogen bonding; (2) the addition of PEG in sPPSU solutions increases viscosity because of enhanced polymer interaction and entanglement; (3) the presence of PEG slows down the phase inversion and suppresses the formation of macrovoids. In addition, with an appropriate amount of PEG in the dope solution, the resultant membrane possesses improved mechanical strength, higher hydrophilicity and permeation properties. To our best knowledge, this is the first paper investigating the roles of PEG in the fabrication of asymmetric membranes cast from sulfonated polymers. This study may reveal the molecular interactions between PEG and sPPSU and provide practical guidelines for membrane fabrication.